Its a Volvo Problem

[Nilao]

From what i see in your picture the red line comes from the trailer brake release valve in the dash. Should only have air supplied to it with the red knob pressed in. The green line should be a supply from the foot valve, always pressurized. The grey line comes from the treadle valve on the dash and the orange line comes from the application port on the foot valve. If you were to remove all from that valve except the green line there should be no air escaping from those lines, with the engine running you will still be able to feel air escaping even without the tanks being at full pressure...if you unhook all those lines and the valve is still venting then its the valve.

Your colors may vary so look and see what lines come from the components of your truck...ie some truck use a orange line from the brake valve release on the truck